Recombination is the central phenomenon underlying genetics and appears to occur in all organisms. The goal of the proposed research is the elucidation of the molecular mechanism of recombination in the lower eukaryote Ustilago maydis. The major focus of the proposed work is on the biochemistry and mechanism of action of rec 1 protein from Ustilago, an enzyme that promotes homologous pairing of DNA molecules in vitro. Our efforts will be directed in several areas. We will continue characterization of the protein using biochemical and immunological means, continue studying the homologous pairing reaction using viral DNA molecules as substrates, investigate the binding of DNA and nucleotides to the protein, and examine the protein in various rec 1 mutants. We also hope to concentrate much of our effort on the role of left-handed Z-DNA in the recombination reaction promoted by rec 1 protein. Experiments planned include measuring quantitatively the Z-DNA generated during the homologous pairing reaction, investigating the preferential binding of Z-DNA to the protein, and studying how the recombination reaction between two DNA duplexes is promoted by Z-DNA stretches. In a final area of study we propose to characterize a second recombinase activity that we have purified from mitotic cells. This activity also promotes synapsis and strand exchange, but is found in heat shocked cells.